1. Field of the Invention
The present invention relates to the recovery of tungsten from its ores, particularly as a compound such as ammonium paratungstate (APT). In particular, the present invention relates to the treatment of an aqueous solution containing sodium tungstate (Na.sub.2 WO.sub.4) and dissolved impurities to recover an aqueous solution of ammonium tungstate via extraction of tungstate values from the aqueous sodium tungstate solution into an acidic organic phase, followed by stripping of the tungsten values from the organic phase into a basic aqueous ammoniacal solution. More particularly, the present invention relates to improvements in the process of stripping the tungsten-rich organic phase with the ammoniacal aqueous solution.
Tungsten is frequently recovered from its ore by a series of steps including alkali digestion of the ore to recover an aqueous solution of sodium tungstate. Following removal of impurities such as silica and molybdenum from this aqueous solution, it is passed through solvent extraction and stripping steps to produce an aqueous solution of ammonium tungstate that should be essentially free from sodium ions and should contain only minor amounts of sulphate ion. In the extraction step, the sodium tungstate solution is mixed under acidic conditions in several stages with a water-immiscible organic phase comprising, for instance, an alkylamine diluted in kerosene, and substantial amounts of tungsten values pass into the organic phase. The aqueous-organic mixture is then allowed to separate into two discrete phases. The mixing and separating operations of the extraction step can be performed in mixer-settler units of conventional design. The tungsten-enriched organic phase is typically washed, and then conveyed to the stripping step.
The stripping step comprises one or more stages each comprising a stripping unit and a phase separation unit such as a settler. In the stripping unit, the tungsten-bearing organic phase is mixed with an aqueous stripping solution of ammonia which also contains some ammonium tungstate in solution. Tungsten values are stripped from the organic phase into the aqueous ammoniacal stripping solution, forming a mixture of the tungsten-depleted organic phase and a tungstate-enriched aqueous solution. The mixture is then separated, for instance by allowing the aqueous solution to settle from the organic phase in the settler. The stripped organic phase can be washed and recycled to the extraction step, and the tungstate-rich aqueous solution is processed for the recovery of, e.g., solid ammonium paratungstate ((NH.sub.4).sub.10 W.sub.12 O.sub.41) (APT) crystals.
In practice, however, the conditions under which conventional stripping techniques have been carried out previously have favored the formation in the stripping unit of solid reaction products, particularly APT, which interfere with the normal operation of the settler. It is desirable that the aqueous and organic phases form two sharply divided layers in the settling stage as quickly as possible, to maximize efficient recovery of tungsten while minimizing contamination of the product, but solid reaction products that are formed in the stripping unit and are carried into the settler interfere with the separation that must be attained between the organic and aqueous phases. Since solids that are carried into the settler generally do not redissolve there, the solids must be dealt with before reaching the settler. Physically removing the solids necessitates additional process time and equipment, and to the extent that the removed material contains tungsten its removal represents a decrease in the amount of tungsten which would otherwise report to subsequent recovery stages. Thus, the formation of APT or other solid reaction products in the stripping unit should be minimized.
Past efforts directed toward minimizing the formation of APT in the stripping unit have met with less than complete success, while imposing restrictive and expensive requirements as to equipment size and as to operating conditions and controls. Thus, there is a need for a process for stripping a high percentage of tungsten values from the organic phase into the aqueous ammoniacal solution quickly while minimizing the formation of solid APT.
2. Description of the Prior Art
Previous techniques for stripping tungsten have dealt with the formation of solids in the stripping unit by increasing the size and/or number of mixing compartments so as to provide enough residence time for the solids to redissolve before the liquid passes to the settler. This approach increases overall process time, and raises costs, for a given amount of production.
Also, the prior art has favored operating the stripping unit with relatively dilute concentrations of ammonium tungstate so as to lessen the proximity to saturation of the aqueous phase with respect to tungsten, and thus minimize formation of APT solids in the stripping unit. These techniques require the operator to sacrifice rate of production as well as flexibility of operating conditions, and they require wasteful commitment of equipment capacity. In particular, the load on the APT crystallizer is increased due to the relatively low WO.sub.3 concentration of the ammoniacal liquor fed to the crystallizer and the larger quantity of water that must be evaporated.
South African published patent application Ser. No. 68-492 shows the inevitability with which the prior art has viewed both the formation of undesired solids in the stripping column and the need for sizing the equipment so as to allow the undesired solids to redissolve in the stripping column. The applicant teaches stripping tungstate from an organic-amine phase by feeding the organic phase into the side of a column agitated by a marine-type propeller lying in a horizontal plane just above the top of the side inlet. The applicant states that upon initial contact of the loaded organic phase with the stripping solution in accordance with the disclosed method there occurs some precipitation of a "white, tungsten-containing compound" which redissolves before reaching the settler stage "providing the column is of sufficient length". It is submitted that this teaching suggests increasing the size and, therefore, the residence time of the stripping unit in order to keep APT solids from passing into the settler.
In the South African application, an aqueous ammonium tungstate strip liquor containing 350 to 370 gpl of WO.sub.3 is formed by contacting the organic phase with a stripping solution having a pH of 10 to 11 and containing 20 to 125 gpl WO.sub.3. The preferred WO.sub.3 concentration in the stripping solution is 100 gpl or less, such as 20 to 40 or 50 gpl; it is believed that the lower WO.sub.3 concentrations are preferred in order to reduce the formation of solid APT in the stripping unit.
U.S. Pat. No. 4,092,400 describes stripping an organic phase containing 100 to 150 gpl of WO.sub.3 with an aqueous solution containing about 1.3 wt.-% ammonia. Solid APT forms on contact between the organic and aqueous phases, and a retention time of at least 10 minutes is required to assure that the APT solids that form are redissolved.
The prior art thus has not recognized the particular combination of conditions under which as tungsten-laden acidic organic stream can be stripped of tungsten values in a short residence time without passage of undesired solids to the settling unit.